extension MapEx on Map<num, num> {
  void append(num key, num? b) {
    final a = this[key];
    if (a == null) {
      this[key] = b ?? 0;
    } else {
      this[key] = a + (b ?? 0);
    }
  }

  void del(num key, num? b) {
    final a = this[key];
    if (a == null) {
      this[key] = b ?? 0;
    } else {
      this[key] = a - (b ?? 0);
    }
  }
}
